As investors, we need to diversify all our investments so it is better to have a little of this and that. It is good to hold BTC and ETH but we should also buy some of the alts because I believe in the never dying advice which is to never put all your eggs in one basket. Doing so will not only make your portfolio look good and diverse but also one or more of them alts will at some point help you cope up in time of losses from other coins.